For the rankings, 22 experts, including nutritionists, dietitians, cardiologists and diabetologists, reviewed 20 popular diet profiles that were developed by reporters and editors at U.S. News and World Report.

But now, proponents of the Paleolithic diet are defending their research and the diet by saying eating like a caveman may be the healthiest thing you ever do.

The eating regimen consists of fruits, vegetables, lean meats, fish and nuts. The dieters are told to avoid salt, sugar, milk and grains, as humans in the Stone Age time did not have access to such foods.

After the rankings giant posted its results two weeks ago, Loren Cordain, a leading voice on Paleolithic diets and professor of health and exercise at Colorado State University, posted an online rebuttal, noting five clinical studies that found the Paleo diet lowered blood pressure and cholesterol and improved insulin resistance. The trials specifically related to the Caveman Diet were small (none of them involved more than 30 people), but Cordain's blog post gained the attention of Paleo diet followers, who then also went online to personally vouch for the eating style.

On the U.S. News and World Report rankings Paleo diet page, more than 3,000 people said the diet had worked for them, while 80 said it did not. Compare that to the No. 1-ranked diet, Weight Watchers, in which 1,883 people said the diet worked for them and nearly 800 people said it did not.

"Whoever was the writer of these reviews had not read the science behind the Paleo diet," Cordain said. "I personally don't like evaluating something and have huge bits of evidence that weren't examined. That's bad science.

"There have now been five clinical trials showing that this diet is a powerful way to normalize health and well-being," Cordain said.

But U.S. News and World Report stood by its ratings.

"For the Paleo diet, additional evidence is needed to show conclusively whether or not it is as effective as some people hypothesize," said Ben Harder, general manager of Health & Science at U.S. News & World Report. "The most relevant studies have been small, as our published review of the Paleo diet indicates. We hope researchers will publish more -- and larger -- studies on the Paleo diet so that health experts, including our expert panel, have more evidence to consider in the future."

Cordain said the science behind the Caveman diet is analyzed by "evolution through natural selection." While he is put off by the term "caveman diet," because it sounds silly, sexist, and makes people thinking of Fred Flintstone and Barney Rubble swinging clubs around, there is an important scientific explanation behind the eating style.

"The environment has sped way ahead of what our genes are adapted to," he said. "We are a good field animal, we're good when we're outdoors and expending energy to get energy, but in the modern world, we've broken that link.

"We're stone-agers misplaced in space-age time."

'Caveman Diet' Receives Strong Backing

Dr. David L. Katz, director of the Yale University Prevention Research Center, was on the 22-person panel. While he did not personally rank the Paleo diet last in the overall weight-loss category, he did raise several issues about what the diet actually means. He noted that our Stone Age ancestors gathered more than hunted by most accounts, and had a mostly plant-based diet.

"The meat our Stone Age ancestors ate is nothing like the meat we eat today," said Katz. "When's the last time you saw a mammoth? I rest my case."

While Katz said the idea behind it is indeed a good one, in this day and age, it could easily be misinterpreted.

"It is the dietary pattern to which we are adapted -- our native diet -- and as such, almost certainly informative about what's good for us," Katz said. "But ... it's easy to turn the concept of Stone Age-style eating into an excuse for hamburgers, and that certainly won't do your health any good."

While Keith Ayoob, associate professor of pediatrics at Albert Einstein College of Medicine in the Bronx, N.Y., said he has "no problem with people eating plenty of lean protein and fresh fruits and vegetables," he did have a problem with the lack of whole grains and low-fat dairy.

"I can't recommend a diet that advocates exclusion of whole foods groups, and foods like low-fat yogurt and milk and beans and whole grains," Ayoob said. "As for dairy, a lifetime of no dairy and you're really risking osteoporosis and low bone density. Paleo man didn't have to worry -- he'd be dead by age 40, but the rest of us would like to hang out for considerably longer and have strength while we do."

But Cordain argued that dairy is not a necessary diet staple, and said questioning its absence is a "kneejerk" response in the medical community. He said 65 percent of people are lactose intolerant, and it wasn't until relatively recent times that humans even began drinking mammals' milk.

"They lived outdoors so they had a lot more sun exposure," Cordain said. "Their blood concentration of vitamin D was likely higher, which enhanced the calcium absorption in their diets.

"We always are thinking more more more calcium, but it's just as much about the amount of calcium we absorb as how much we're putting in our bodies," he said. "This notion that all you have to do is put more into your system is incorrect."

Dr. Lynda Frassetto, professor of medicine at University of California at San Francisco, has conducted research on the Paleo diet. While she did not research the weight loss possibilities of the Paleo regimen, she did say that people on a strict Paleo diet lowered their cholesterol and lipids and their insulin improved.

Paleo diet advocates argue that obesity and diabetes didn't occur 10,000 years ago, so the eating habits and lifestyles are worth exploring for a modern-day approach to these epidemics.

While Ayoob argues that the Paleolithic man didn't live long enough for such problems to occur, as most died around 35 or so, they did eat locally and perform high amounts of physical labor, things Ayoob said he could get behind.

"Of course, if people did that much physical activity, they probably wouldn't be obese in the first place," Ayoob said.

"Physical activity is often a deciding factor for much of our chronic health problems," he said. "Much of type 2 diabetes is obesity related and instead of focusing only on diet, we need to focus on activity also, because physical activity can affect insulin resistance, body weight, cholesterol and triglyceride levels, joint health, and so on. Without that, there can be no balanced lifestyle."